1. Field of the Invention
The present invention relates to a magnetoelectric element and a magnetoelectric apparatus having the same and, more particularly, to a novel magnetoelectric element for converting a magnetic field into an electric capacitance utilizing a Lorentz force acting on a current in the magnetic field and outputting the converted electric capacitance and a magnetoelectric apparatus having the same.
2. Description of the Related Art
A Hall element which utilizes a Hall effect is well known as a magnetoelectric element. FIG. 1 shows the Hall element. Hall element 1 comprises Cr-doped semi-insulating GaAs substrate 2, cross-shaped magnetoelectric layer 3 obtained by doping an N-type impurity in substrate 2, and four metal electrodes 4a, 4b, 4c, and 4d formed each at four distal ends of a cross portion of layer 3. A current is externally supplied to electrodes 4a and 4b. In this case, current I is supplied in a direction from electrode 4a to 4b. In addition, a magnetic field is vertically applied downward with respect to the drawing surface. Current I is subjected to the Lorentz force caused by the magnetic field and deflected toward electrode 4c. Carriers of the current in N-type magnetoelectric layer 3 are electrons, and the electrons as carriers are deflected toward electrode 4d. Therefore, an electromotive force in which electrode 4c has a positive polarity and electrode 4d has a negative polarity is generated. Since the intensity of the magnetic field is proportional to the Lorentz force acting on the electrons, this electromotive force is proportional to the intensity of the magnetic field. Therefore, the Hall element constitutes a magnetoelectric element capable of converting a magnetic field into an electromotive force.
In FIG. 1, output voltage V generated between electrodes 4c and 4d of the Hall element to which the magnetic field is applied is represented by the following equation: EQU V.sub.H =K*RdIB (1)
where K* is a constant called a specific sensitivity, Rd is the electric resistance of a magnetoelectric layer between electrodes 4a and 4b, I is the current between electrodes 4a and 4b, and B is the magnetic flux density. In equation (1), the magnitude of voltage V.sub.H is changed in proportion to the magnetic flux density. Therefore, a signal of voltage V.sub.H is obtained as an analog signal.
In addition, in equation (1), voltage V.sub.H is theoretically not generated when B=0, i.e. when no magnetic field is generated. However, a certain noise voltage is actually generated. This is called an off set voltage. Therefore, voltage V.sub.H actually includes off set voltage V.sub.H0, and equation (1) can be rewritten as follows assuming that an actual output voltage is represented by V.sub.HM : EQU V.sub.HM =V.sub.H +V.sub.H0 =K*RdIB+V.sub.H0 ( 2)
In equation (2), a condition of V.sub.H &gt;&gt;V.sub.H0 must be satisfied in order to detect a minimum magnetic field. V.sub.H0 /V.sub.H is called an off set ratio, and a product to be actually used must satisfy an off set ratio of 10%. For example, in a Hall element in which V.sub.H =100 mV is obtained when B=1000 gauss, a minimum detection limit of the magnetic field is given as follows: EQU 1000 gauss.times.10%=100 gauss (3)
Since this value represents a theoretical limit, an actual limit must be twice to three times the theoretical limit in consideration of a practical accuracy. Therefore, an actual minimum limit is 200 to 300 gauss.
As described above, the Hall element has a comparatively large minimum detection limit. In order to further improve the detection sensitivity, V.sub.H0 must be minimized. However, the presence of V.sub.H0 is partially caused by a size precision and nonuniformity of component materials in the manufacture of the Hall element. It is difficult to economically reduce inconvenience caused by these factors in the manufacture. For this reason, a demand has arisen for a magnetoelectric element with a high detection sensitivity which can be used in place of the Hall element.
In addition, along with recent progresses in IC techniques, information is generally, digitally processed and controlled. In order to digitally process an output from the Hall element, an output signal from the Hall element must be converted into a digital signal. In order to digitize the output from the Hall element, the output must be analog/digital (A/D)-converted, and a so-called preamplifier for amplifying the output from the Hall element to an output having a predetermined magnitude is necessary. For these reasons, an electric circuit is complicated, and therefore realization of an IC arrangement becomes difficult and an IC becomes expensive.